The present invention pertains to the general field of printing and in particular to the engraving of gravure cylinders.
Gravure is one of the main processes employed by the printing industry, with billions of copies of gravure-printed magazines being produced annually. Gravure printing is also employed extensively in the packaging industry.
In the gravure printing process ink is transferred to the medium, typically paper or plastic, via metal printing cylinders that are normally several meters long. The gravure process transfers ink from small wells or cells that are engraved into the copper- and chrome-plated surface of these cylinders, while the cylinders are mounted on the printing press. Each cylinder is rotated through a fountain of ink and the ink is wiped from those areas of the cylinder-surface that have no gravure-impressions by a doctor blade. The inverted pyramid-shape or cup-like shape of each gravure-cell holds the ink in place as the cylinder turns past the doctor blade.
The cylinder cells are the most important part of the gravure printing process. The quality of the printed image is dependent on the size, shape and depth of the cells.
The width of the cell refers to how wide the cell is in the cross direction. The depth is how far below the surface the cell extends. The wall is the barrier between the cells and is used to support the doctor blade. The top of the cell wall and the un-engraved areas of the cylinder are commonly referred to as the land. The opening is described by the shape and cross sectional area. The bottom of the cell can be flat, or nearly flat, or inverted pyramid shaped.
Various techniques are employed to engrave gravure-cylinders. Cells can be chemically etched or electromechanically engraved. More recently laser-engraving has become available. Yet more recently electron-beam-engraving has been evaluated with a view to its use in gravure engraving.
Different methods exist to chemically etch gravure cylinders. The traditional chemical etching method, employing carbon tissue, leads to a cylinder that has cells of equal area, but differing depths. The subsequently developed direct transfer technique produces the opposite relationship in that the cells all have the same depth of the order of 20 to 25 microns, but their areas differ. Cell-wall widths are typically of the order of 5-10 microns and etching times are of the order of 3 to 5 minutes.
Electromechanical engraving is the most common method of cylinder imaging today and is a direct result of advances in electronic technology.
Once the image information has been scanned and digitized it is processed for the engraving section of the machine. The objective of the engraving process is to produce cells which, when printed, will duplicate the density of the desired image. The very small volume of ink must be controlled within the engraved cell volume.
The tool used for electromechanical engraving is a diamond stylus of triangular cross section that engraves an inverted pyramid. The digital processed image information is converted to an electronic vibration that produces a mechanical motion in the diamond stylus. The darker the desired image the deeper the diamond penetrates into the copper. The large cell will carry more ink and produce more density. Conversely, if a light tone is desired, the diamond makes only a slight cut into the copper. The cells are cut at a typical rate of 8000 per second, but systems have been demonstrated engraving up to 20,000 cells per second. After engraving the cylinder is plated with chrome for durability.
There are four basic cell structures formed during electro-mechanical engraving. They are compressed, elongated, normal and fine. By using these alternately shaped cells, color process printing becomes possible. The size and position of the cells begin to form a line screen image. This screening effect allows for the successful combination of the four process colors.
Due to the high cost of the diamond stylus and the processing the finished cylinder is a very expensive and significant part of the gravure process. There has therefore been considerable effort devoted to developing lower cost routes to gravure engraving.
Information technology has transformed printing to a very great extent. Since design and layout are now normally conducted electronically, the manufacturers of printing equipment are developing new systems that are fully compatible with the speed, precision, and sustained accuracy of computers. The general aim is to shorten processing times without deviating from the rigorous quality standards demanded by the end users. The engraving of the gravure cylinder and its subsequent plating with chromium for protection, is a time consuming task, however, as a single head precision mechanical engraver takes at least ten hours to complete a drum. There was and is a clear market demand for quicker alternatives.
In response to the aforementioned challenge, there has been much attention devoted to the idea of replacing the diamond styli with an energy beam. Concepts for gravure engraving using electron beams were proposed in the 1960""s. During the decade of the 1980""s there was considerable experimentation with both laser and electron beam engraving, but it proved unsatisfactory with the technology then at hand.
In the early 1990s, more progress was made in the field of indirect laser gravure. The copper roller received an even coating of a substance that was removed by a beam from a modest 60 W laser. The actual inkwells were then created in parallel by chemically etching the roller before it was chromium plated. Though this indirect laser engraving produced cells that were hemispherical, the optimal shape for ink-retention, it was not ideal in its application because the etching stage could not be fully controlled at a reasonable cost. During the decade of the 1990""s there were further developments in which the direct laser-engraving of the cylinder was addressed using 400 Watt lasers. This approach succeeded in generating up to 140,000 inkwells per second, with the walls between the cells being just a few microns. It took less than 15 minutes to complete a square meter of drum surface engraving. Here again, the hemispherical well-shape allowed the wells to be only two-thirds of the depth normally required with diamond-stylus engraving.
Against this background, there is therefore scope for addressing the use of electron beams as a means of engraving the gravure cylinder. Electron beam systems of practical power levels can only function within vacuum. Previous effort within industry consisted of encasing the entire system in vacuum. This leads to grave practical problems and increases cost.
Alternative concepts revolved around evacuating only the minimum of volume surrounding the electron gun and the area of the gravure cylinder to be engraved. However, these approaches involved using various mechanical seals to maintain the vacuum while the gravure cylinder rotates against the seals. This generic solution suffers from the fact that no mechanical sliding seal can conform well enough to the surface of the engraved gravure cylinder to maintain adequate vacuum for the high-energy electron beam, particularly if the seal is directly to atmosphere.
Electron-permeable membranes have been suggested, but these mechanically sensitive structures, while very useful in laboratory circumstances and for low-intensity beams, are ill suited to the industrial conditions that pertain to gravure printing. They also are not adequately permeable to larger charged particles.
The problem of maintaining vacuum as the engraving process approaches the ends of the gravure cylinder has also been previously addressed via various mechanical arrangements that involve fitting extensions to the gravure cylinder.
In accordance with the present invention a gravure cylinder is engraved by means of an electron beam which is modulated to create upon the surface of the gravure cylinder the desired gravure cells, the required vacuum being maintained only in a limited volume around the electron gun by the use of a conformal high vacuum ferrofluid seal that is substantially free of mechanical friction.